98 Prof. R. Shida on a new Instrument for recording the 



the strength and direction of a varying electric current. In 

 view of this, I have designed and constructed an instrument 

 the description of which I have now the pleasure of communi- 

 cating. In order that an instrument may continuously record 

 a varying electric current, it is necessary that it should fulfil 

 the two following conditions : — 



1. That the motion of the needle of the galvanometer (which 

 is a part of the instrument) be such that the same position of 

 the needle always corresponds to the same strength of current, 

 that is to say that the motion be non-oscillatory. 



2. That the position of the needle of the galvanometer at 

 any moment be recorded. 



I shall first explain generally how these two conditions are 

 satisfied in the new instrument I am going to describe. 



As regards the first condition. This condition is satisfied 

 by having a galvanometer whose needle consists of a coil of 

 fine wire suspended in a powerful magnetic field, after the 

 manner of the Siphon Recorder of Sir William Thomson. It 

 is easy to show mathematically that in the case of an ordinary 

 galvanometer, which consists of a magnetic needle suspended 

 inside, or in the neighbourhood of a coil of wire, this condition 

 cannot conveniently be fulfilled without diminishing its sensi- 

 bility. On the other hand, in the case of a galvanometer 

 consisting of a coil hung in a strong magnetic field as above 

 described, it is easy to obtain a great sensibility and, at the 

 same time, a non- oscillatory motion of the needle, as will be 

 seen from the following investigations : — 



Let a be the angle of deflection of the coil at any time t, 

 and let T be its period of oscillation when no current is 

 circulating through it ; then we have for the equation of the 



motion ^ ^!__n 



dt 2 + T 2 -0. 



But when a current circulates through the coil, the equation 

 of the motion will be altered owing to a retardation of the 

 motion due to the current induced in the coil. Let us consider 

 the magnitude of this retardation. If I be the intensity of 

 the magnetic field which the coil occupies at time t, A the 

 area included in all the turns of the coil, and if we neglect 

 the self-induction of the coil on itself (which I think we can 

 confidently do) ; then, plainly, N, the number of lines of force 

 which pass through the coil at time t } is 



N-IAsina ; 



hence aN TA da 



Tt =IAcos «df 



